Auckland Council's three-decade stormwater plan slammed

Auckland Council is seeking a 35-year consent to keep discharging stormwater into the environment.

The application seeks permission for "both the diversion of stormwater through the public stormwater network and overland flow, and the discharge of stormwater to the environment".

But an environmental researcher says stormwater is "just as contaminating" to the urban environment as sewage.

"It carries zinc, lead, oils, diesel, animal waste and anything that's on the roads," Auckland University researcher and former president of the Manukau Harbour Protection Society John McCaffery told RNZ on Thursday.

"The age of many of the older consents is such that they no longer represent current best practice, may not be relevant to the stormwater infrastructure needs of the area and may even be counter-productive to achieving stormwater management outcomes," said Healthy Waters resource management team manager Katja Huls.

The city's beaches are often forced to close due to contamination from sewage and stormwater discharge. Last month, heavy rain caused 40 beaches to be deemed 'high-risk' because of stormwater overflows, and six were closed due to sewage.

The council's 377-page application for consent provides for a six-yearly review to ensure it "remains current as community aspirations and best practice continue to evolve".

"The council holds all the cards, it decides whether it does a review, it decides if issues raised are reasonable."

Ms Huls told Newshub the consent isn't a given - it would be assessed "in the same way as they would a private consent application" - and it includes a requirement for environmental monitoring.

"Monitoring to date has shown there are generally decreasing trends for copper and lead; while zinc has increased in some areas and decreased in others, and sediment contamination remains a concern. By analysing trends we can ascertain which contaminants we need to focus on and what methods could be used to manage them."

And while the 35-year length of the consent application is the maximum it can legally seek, Ms Huls said it is required in order to provide certainty.

"Projects can have a long lead in time, and the intent is to work towards clear and consistent expectations."

Ms Huls said it can be hard to predict where stormwater would be discharged into the environment thanks to the hodge-podge of previous standards and the growing effects of climate change.

"When the storm exceeds the capacity of the network, stormwater will also flow overland and flow into the receiving environment via natural low-points though overland flow paths.

"We can't control the volume of stormwater that discharges on any given day because the network is gravity-fed. Rather, we design it to be large enough to convey a 10-year... storm taking into account climate change. However, older parts of the network may have less capacity due to previous engineering design standards and rainfall predictions."

Presently work is being done to improve overflow hotspots in St Mary's Bay and Picton Rd in Howick.

The application can be read in full on the Auckland Council website. Submissions have to be made by March 20, after which Auckland Council itself will decide whether it has permission to go ahead with the plan.